Due to the recent growing demand for space saving and energy saving a display panel has been used instead of the cathode-ray-tube image display device (hereinafter called “CRT”). For example, a plasma display panel (hereinafter called “PDP”) and a field emission display (hereinafter called “FED”) are widely used.
In the above described PDP and FED, two pieces of glass plates having electrodes in their surfaces are used for the front surface and the rear surface with said surfaces with the electrodes facing diagonally to each other. In the PDP, normally, electrodes are created in a strip shape on the rear surface of the glass plate and barriers made of glass are placed among said electrodes. Also, electrodes on the front surface of the glass plate are created in a stripe shape in a direction perpendicular to the electrodes on the rear surface. These electrodes on the front and rear surfaces are usually coated with a glass layer called dielectric layer. A display cell is determined by said front surface glass, rear surface glass and the barriers and a fluorescence material placed on said display cell is displayed by emitting light by the plasma discharge among the electrodes.
Also, in the FED, electrons are discharged by pixel from the electrode (cathode) on the rear surface to the electrode (anode) on the front surface and crashed against the fluorescence material which is coated on the anode thereby displaying the image. Normally, the cathode is created on the dielectric layer which is made on the glass plate and the electrodes other than the cathode are coated with the dielectric layer. This dielectric layer becomes an insulating film which prevents the electric discharge by coating the electrodes and their lines. The dielectric layer is made by a thick film technique which generally uses glass paste and the like.
Therefore, it is sought that the glass used for the barrier and the dielectric layer of the PDP and the dielectric layer of the FED is manufactured at a high degree of accuracy. For example, a patterning of the glass by photo lithography is applied. Normally, this patterning by photo lithography uses an optical patterning glass material such as photosensitive glass paste or photosensitive green sheet wherein an optical-insoluble type photosensitive resin or optical-soluble type photosensitive resin is mixed with a glass in a powder form (powdered glass). For example, when the dielectric layer of the FED is created by using a photosensitive green sheet which uses an optical-insoluble type photosensitive resin, said photosensitive green sheet is laminated on a glass substrate and a photo mask which is made so that light is transmitted only to the dielectric layer creating part, is further laminated. Then, by irradiating light with a wavelength of 365 nm which is called i line by using a mercury lamp, the photosensitive resin on the dielectric layer creating part is exposed to the light making said resin insoluble, removing the soluble part by using a solvent and sintering the sintering part.
Conventionally, the glass substrate on the front and rear surfaces of the PDP and the FED uses a soda lime glass and glass with a high distortion point wherein the content of alkali is decreased. Therefore, it is sought that the optical patterning glass material can be sintered at lower temperature and that the glass used for said optical patterning glass material has low softening temperature (softening point). As for the glass with a low softening point, for example, Japanese laid-open No. H08-119725 discloses that a glass with a low melting point which has PbO as the main element is used for the barriers of the PDP.
Recently, stemming from an increasing awareness of environmental protection, lead-free products are desired in electric equipment and electronic devices. Japanese laid-open No. H11-92168 discloses that lead-free alumino-borosilicate glass is used instead of a glass having PbO as the main element.
However, it is difficult to sufficiently reduce the softening point of the lead-free alumino-borosilicate glass. With the glass disclosed in Japanese laid-open No. H11-92168, it is difficult to obtain a glass with a low softening point which is suitable for being sintered on a soda lime glass or glass with a high distortion point wherein the content of alkali is decreased.
Japanese laid-open No. 2003-128430 discloses that, by using a lead-free bismuth glass which can easily obtain a glass with a lower softening point than that of lead-free alumino-borosilicate glass, the barriers and dielectric layer of the PDP are created. However, when a powdered glass is created by using the lead-free bismuth glass and said powdered glass is mixed with a photosensitive resin to create a optical patterning glass material such as a photosensitive glass paste and photosensitive green sheet, the degree of accuracy of the patterning is decreased. For example, when an optical-insoluble type photosensitive resin is used as the photosensitive resin, the patterns tend to be thicker than those of a photo mask. On the other hand, when an optical-soluble type photosensitive resin is used, the patterns tend to be thinner than those of the photo mask.
In other words, in the optical patterning glass material used for creating barriers and dielectric layer of the PDP and the dielectric layer of the FED, the conventional lead-free bismuth glass has a disadvantage of decreasing the degree of accuracy of the patterning of the optical patterning glass material.